The invention relates in general to acoustic sensors and in particular to the use of acoustic sensors to identify underground objects.
More than 100 million landmines have been deployed throughout the world, resulting in tens of thousands of people dead or maimed every year. U.S. soldiers are deeply immersed in peacekeeping, humanitarian, and military operations in areas of the world cluttered with mines. There are diverse technologies for detecting mines and explosive ordnance with varying degrees of performance, cost, and practicality.
Magnetometers are used to detect ferromagnetic objects such as mines. However, many mines are made of plastic with minimal metal components, and metallic debris often confounds magnetometer detections. The use of thermal imaging for mine detection relies on the mine releasing or storing thermal energy at different rates than its surrounding. Explosive vapor sensors are effective, but are big and slow. Thermal neutron activation for bulk explosive detection has practical limits. X-ray backscatter, mm-wave emissivity, chemical/biosensors, ultra-wide and mm-wave radars are also promising (See Gros and Bruschini, Int""l Symp. on Meas. and Control in Robotics, Brussels, May 1996).
Ground penetrating radar emits electromagnetic waves and monitors the reflections from the soil caused by dielectric variations from underground objects. However, non-metallic mines are only detectable if their dielectric properties strongly contrast with their surrounding. Target-specific resonances can be present in the reflected signal (See Peters, Daniels, Young, xe2x80x9cGround Penetrating Radar as Subsurface Environmental Sensing Toolsxe2x80x9d Proc. IEEE, Vol. 82, No. 12, December. 1994, pp. 1802-1822).
Seismic sensors can also detect resonances for discrimination between metal, plastic, wood, and rocks. Seismic echo-rangers observe mine echoes via generation and detection of scattered Rayleigh and/or surface compressional waves reflecting off a buried mine and returning to a sensor array (See BBN Systems and Technologies Corp., xe2x80x9cFeasibility of Acoustic Landmine Detection: Final Technical Report,xe2x80x9d Report No. BBN-TR-7677, May 19, 1992).
Acoustic (ultrasonic) imagery is commonplace in medicine. Broadband acoustic detection is effectively employed in underwater warfare and the detection of underwater mines buried in sea-bottom silt. Reflections at material discontinuities, as well as mine dimension, shape, materials, and depth contribute to the distortion of the induced and resultant sound field. These effects, often subtle modifications to amplitude, phase, and frequency, are easily monitored to extract information relating to an object within its surroundings.
Acoustic systems are capable of good penetration through very wet and heavy ground, such as clay, xe2x80x9cbut are likely to experience problems at the air-ground interface.xe2x80x9d (See Bruschini and Gros, xe2x80x9cA survey of Current Sensor Technology Research for the Detection of Landmines,xe2x80x9d Int""l Workshop on Sustainable Humanitarian Demining (SusDem""97), Sep. 29 -Oct. 1, 1997 Zagreb, Croatia). Successful imaging with 15 MHz was conducted on a mine submerged slightly underwater, like deployed in rice fields. Such high frequencies will not normally penetrate the ground, and more appropriate frequencies and coupling should be used. Transmitting 3 kHz pulse bursts into the ground has permitted detecting objects down to 12 inches, and shown that rock-reflected signals exhibit irregular axes of reflections (See Morita. xe2x80x9cLand Mine Detection System,xe2x80x9d TRW Final Report AT-73-2, Feb. 23, 1973).
The introduction of soliton-like shock waves into the ground showed they had weak interaction with the ground, which causes minimal dispersion, and can provide much information from mine reflected energy (See Sen, Physical Review Letters, vol. 74, p. 2686-2689, 1995 and Physical Review E, vol. 54, pp. 6857-6865, 1996). Millisecond acoustic burst/impulse techniques provide advantages over continuous wave (CW) techniques. Return pulse gating allows interpretation of travel path and spectral modifications, since the pulse contains typically 200 Hz to 20 kHz data (See Rogers and Don, xe2x80x9cLocation of Buried Objects by and Acoustic Impulse Technique,xe2x80x9d Acoustics Australia 22 5-9, 1994). A significant problem lies in isolating small object pulses from other, often dominant, signals, and coping with ground contours and irregularities (See Don, xe2x80x9cUsing Acoustic Impulses to Identify a Buried nonmetallic Object,xe2x80x9d Abstract 2aPA3, 127th Meeting of the Acoustical Society of America, May 1994). CW and broadband acoustics may impart more energy to better induce structure resonances.
A US Army study found that disturbed soil covering a mine absorbed acoustic energy while the surrounding undisturbed soil reflected the acoustic energy. Where the acoustic energy was absorbed, the ground vibrated at seismic frequencies that depended on the acoustic input, soil properties, and on the mine (See More, Dilworth, Lewis, Wesolowicz, and Stanich, xe2x80x9cAcoustic Mine Detection,xe2x80x9d Daedalus Enterprises Final Report, Feb. 7, 1990). This implies that complementary sensor technologies, such as passive/active acoustic/seismic can enhance detection and identification through sensor fusion.
The present invention employs acoustic array techniques to localize buried objects and interpret the landmine""s environment. One embodiment of the present invention is a low-cost, hand-held mine detector that rolls or slides across the ground, suitable for a soldier to inspect and clear, for example, a two-foot wide path for him to walk. In some embodiments, the invention incorporates data from seismic and electromagnetic sensors to enhance detection and reduce false alarms. Acoustic coupling and imaging can also aid in the nondestructive evaluation of materials and structures.
In accordance with the invention an apparatus for detecting an underground object comprises a container in contact with a ground surface; a medium disposed in the container; at least one acoustic sensor disposed in the medium in the container, for detecting acoustic noise; and an output device connected to the acoustic sensor. The apparatus further comprises at least one acoustic source that emits acoustic noise. The medium is at least one of liquid and gel. At least a portion of the container that contacts the ground surface is substantially acoustically transparent. The at least one acoustic source may be disposed in the medium in the container.
The portion of the container that contacts the ground surface is made of a substantially flexible material such that the portion of the container that contacts the ground surface substantially conforms to a contour of the ground surface. The substantially flexible material is one of rubber, polyethylene, polyvinylchloride, vinyl and a plastic material. The medium is one of water, oil and oil well drilling mud. The output device comprises a visual display, an auditory device or a tactile device.
In one embodiment the container is a roller having a generally cylindrical shape, the roller including a shaft that passes through the roller wherein the at least one acoustic sensor is mounted on the shaft. At least one acoustic source that emits acoustic noise may also be mounted on the shaft. A handle may be attached to the shaft for moving the roller across the ground surface. The acoustic noise is one of swept sine impulsive, broadband and continuous wave.
Preferably, an acoustic impedance of the medium is substantially the same as an acoustic impedance of material around the underground object.
The apparatus may further comprise a data processor connected between the at least one acoustic sensor and the output device. The data processor compares data from the at least one acoustic sensor and the at least one acoustic source. The data are compared for variations in at least one of phase, amplitude, frequency, time of arrival and echoes.
The apparatus may further comprise a rear wheel assembly attached to the handle, for decreasing loading of the roller on the ground surface.
In one embodiment, the apparatus further comprises a calibration bar including a reflective surface, the reflective surface being suspended beneath the at least one acoustic sensor and the at least one acoustic source for calibrating the at least one acoustic sensor and the at least one acoustic source.
In a preferred embodiment, the apparatus further comprises a collection chamber assembly mounted on the shaft inside the roller at one end thereof, the collection chamber assembly comprising a chamber with openings formed therein for collecting and releasing higher density medium.
The shaft may include a bearing portion and a shroud portion, the at least one acoustic sensor and the at least one acoustic source being mounted on the shroud portion. An acoustic absorber may be mounted on a top of the shroud portion.
In another embodiment, the shaft includes a transverse portion and at least one longitudinal portion, the at least one longitudinal portion being substantially parallel to a direction of movement of the roller the at least one acoustic sensor and the at least one acoustic source being mounted on the at least one longitudinal portion.
In a preferred embodiment, the container comprises a rigid top plate, the at least one acoustic sensor being mounted on a bottom of the rigid top plate.
In another embodiment, the apparatus comprises a plurality of rollers, the plurality of rollers comprising two end rollers and at least one interior roller; a transverse member; two end arms connected at one end to an end roller and at the other end rigidly connected to the transverse member; at least one rocker arm connected at one end to the at least one interior roller and at the other end rotatably connected to the transverse member; and a handle connected to the transverse member.
The apparatus may further comprise a global positioning system attached to the apparatus.
In another embodiment the container is a roller having a generally cylindrical shape, the roller including two end caps, a support structure mounted between the two end caps, at least one mounting plate attached to the support structure for receiving the at least one acoustic sensor, a pair of shaft hubs attached to external surfaces of the end caps and a transmitter attached to one of the pair of shaft hubs.
The invention further provides an apparatus for inspecting a pipe comprising at least one generally cylindrical roller having a substantially acoustically transparent portion that contacts an interior of the pipe; a shaft that passes through the at least one generally cylindrical roller; at least one acoustic sensor mounted on the shaft; a medium contained in the roller, the at least one acoustic sensor being immersed in the medium, the medium having an acoustic impedance substantially the same as an acoustic impedance of the pipe; a central support arm and a central support ring rigidly connected to the central support arm; a pair of roller support arms connected at first ends to ends of the roller shaft and at second ends pivotally connected to the central support ring; a pair of expansion springs connected at first ends to the central support arms and at second ends to the pair of roller support arms, respectively, wherein the expansion springs force the roller against the interior of the pipe; one of a radio transmitter and a data logger mounted on the central support arm and connected to the at least one acoustic sensor; and means for moving the at least one roller through the pipe.
Another aspect of the invention is a method of detecting an underground object comprising receiving at least one of acoustic noise emanating from and acoustic noise reflected from the underground object with at least one acoustic sensor immersed in a medium, the medium being disposed in a container in contact with a ground surface; converting the received acoustic noise to electrical signals; using an output device, converting the electrical signals to a form that can be sensed by a human to determine if the underground object has been detected. A portion of the container in contact with the ground surface is substantially acoustically transparent. An acoustic impedance of the medium is substantially the same as an acoustic impedance of material around the underground object.
The method further comprises conforming the portion of the container in contact with the ground surface to contours of the ground surface. The method further comprises processing the electrical signals with a data processor.
In one embodiment, the method further comprises converting the electrical signals to the form of one of a visual display, auditory cue and tactile cue.
Another aspect of the invention is an apparatus for analyzing a material comprising a container in contact with a surface of the material; a medium disposed in the container; at least one acoustic sensor disposed in the medium in the container, for detecting acoustic noise; and an output device connected to the acoustic sensor. The apparatus further comprises at least one acoustic source that emits acoustic noise.
Yet another aspect of the invention is a method of analyzing a material comprising receiving acoustic noise from the material with at least one acoustic sensor immersed in a medium, the medium being disposed in a container in contact with a surface of the material; converting the acoustic noise to electrical signals; using an output device, converting the electrical signals to a form that can be sensed by a human.
In one embodiment, the material is soil and the method further comprises analyzing the soil for at least one of soundspeed, porosity, density and water content.
Further objects, features and advantages of the invention will become apparent from the following detailed description taken in conjunction with the following drawing.